The reducing property of magnesium vapor for purifying metals from metal oxides, sulfides and the like is well known as represented by Hivert, et al., U.S. Pat. No. 2,881,067 which teaches the production of powder metals from oxides by reaction with magnesium vapor produced by heating elemental magnesium.
Similarly, Shekhter, et al., U.S. Pat. No. 6,171,363 and U.S. Pat. No. 6,558,447, employ elemental magnesium mixed with tantalum and niobium oxides and directly heated to about 1000° C., thereby generating gaseous magnesium in direct contact with the metal oxides to reduce the oxides to pure metal.
The problem with the prior art processes for reduction of metal oxides by magnesium vapor is in obtaining the elemental magnesium for heating to provide magnesium vapor.
Since the early part of the 1900s, Hansgirg and others have demonstrated that carbon can be used to liberate magnesium vapors from magnesium oxide at high temperatures to produce pure magnesium metal for subsequent use such as in the above noted reduction of metal oxides. Magnesium oxide is reacted with carbon, usually at or above 2200 degrees K. Carbon has an increasing affinity for oxygen at such elevated temperatures, while magnesium's bond with oxygen becomes less stable with increasing temperature. The result of this carbothermic reduction of magnesium oxide is production of a mixture of magnesium gas and carbon monoxide gas which are then separated to yield elemental magnesium for other uses.
Such separation processes are represented by Mod, et al., U.S. Pat. No. 3,560,198, in which magnesium and carbon monoxide vapors produced by typical carbothermic reduction of MgO ores are passed through a bed of metal carbides at temperatures of 1500 to about 1850° C. At those temperatures, the carbon monoxide reacts with the metal carbides to form metal oxides and carbon. However, Mod, et al., teach that the magnesium vapors do not react with either the carbides or the resulting oxides and that the elemental magnesium is separated out for recovery.
Also, Diaz, et al., U.S. Pat. No. 5,782,952, provides a continuous process for the production of elemental magnesium from magnesium oxide and a light hydrocarbon gas reacted at a temperature of about 1400° C. or greater. The resulting product stream is continuously quenched to separate elemental magnesium for use elsewhere.
However, problems with the process to yield magnesium metal occur due to magnesium's greater affinity for oxygen when the mixture cools, producing a back-reaction to magnesium oxide and carbon. Attempts to rapidly quench the mixture to minimize the opportunity for back-reaction and produce magnesium metal have a tendency to yield magnesium powder which is hazardous in large-scale environments, as noted by Diaz, et al. Other attempts used to refine the carbothermic method to produce magnesium have included rapid quenching with hydrocarbons, liquid magnesium, inert gases, and the use of supersonic nozzles to accelerate the quenching process. Filtering the carbothermic product mixture through metal carbides to separate the magnesium from the carbon monoxide has also been tried as taught by Mod, et al.
None of these prior processes teaches the direct utility of use of the gaseous magnesium and carbon monoxide mixture produced by carbothermic reduction of magnesium oxide with carbon as a reducing agent for production of metals from metal oxides, hydroxides, sulfides or polyatomic compounds, thereby avoiding the necessity of first separating the elemental magnesium and preparing it for use as a source of magnesium vapor.
The present invention takes advantage of the stability of carbon monoxide at the carbothermic reduction temperatures, such that the high-temperature mixture can be used to reduce raw materials and recover other non-magnesium substances. Carbon monoxide at approximately 2200 degrees K is stable and can be considered an inert gas in a carbothermic mixture, so that the strong reducing nature of magnesium vapors can be employed with other substances needing reduction. Examples of such substances which can be reduced using the present method include chromium oxide, polyatomic chromium compounds containing oxygen, manganese oxide, polyatomic manganese compounds containing oxygen, and zinc oxide, hydroxide, sulfide, or polyatomic zinc compounds containing oxygen or sulfur.
The present invention takes advantage of the stability of carbon monoxide at carbothermic temperatures such that the high temperature gaseous mixture of magnesium and carbon monoxide can be used to reduce such oxides, hydroxides, sufides and polyatomic compounds to greater purity than prior methods.
For example, most chromium is currently made by the carbon reduction of chromium III oxide, or Cr2O3.Cr2O3(c)+3C(c) at 2200° K yields Cr(l)+3COThe Gibbs Free Energy values for this reaction are−562 KJ/mol+0 - - - 0+−908 KJ/mol.A negative Gibbs Free Energy means that reduction of metal oxides to the metallic state is favorable.
Unfortunately, in actual practice, small amounts of free accumulated carbon react with the chromium to produce some contaminating chromium carbide, Cr7C3(c), which is quite stable at −221 KJ/mol. This means that very-low carbon chromium is usually made by reduction with aluminum, the aluminothermic reduction, or by electrolytic methods such as electrolytic deposition from a chromium-alum electrolyte made from high carbon ferrochromium, or, in the recent FFC process, electrolyzed at 1200° K using CaCl2 liquid, producing a sponge metal. In the latter process, the metal sponge must be heated to 2200° K to melt the chromium into ingots anyway. Because of the materials required and added steps, these are more expensive processes than this proposed invention.
Similarly, manganese is currently produced carbothermically, using carbon granules interacting at high temperature with manganese II oxide, MnO, the heat decomposition product of Mn3O4. This method, similar to the chromium situation discussed above, is complicated by the formation of manganese carbide impurities. Additional reaction with MnO is usually done to try to eliminate as much of the carbon from the manganese as possible, but some impurities remain. Another method to produce manganese is by electrolysis of manganese sulfate solutions which raises environmental problems.
Zinc has been commonly produced by roasting the main ore, ZnS, in air to form ZnO and SO2. The ZnO is then carbothermically reduced to Zn, CO, and CO2 gases. However, this process presents serious problems with air pollution and the back-reaction between the CO2 and Zn. Most zinc is now made electrolytically where ZnS is oxidized to ZnSO4, either by treating roast-produced ZnO with sulfuric acid (the acid being generated by the oxidation of the ZnS), or, to avoid the pollution-plagued roasting, the ZnS is directly treated with sulfuric acid and oxygen under pressure, with deposited free sulfur used to generate the sulfuric acid. The ZnSO4 is then electrolyzed in an aqueous environment.